1. Field of the Invention
The invention relates to a joint for an intermediate metal conduit, and, more particularly, to a joint wherein sections are assembled by using a coupling ring which will permit sections to swivel with respect to one another during coupling and to be disassembled without axial longitudinal displacement of the conduit sections.
2. Prior Art
Until very recently, there were on the market only two kinds of steel conduit for electrical cables; one of these was called rigid pipe and the other was called light walled conduit.
The rigid pipe has been available for over one hundred years. It is a thick walled pipe that is conventionally used for the transmissions of gas and water. It also is used for sheathing electric wires in buildings in which case it is frequently embedded in concrete. As a sheathing for electrical cables, rigid pipe is used where the pipe has to be very strong, or where the pipe may be subjected to a heavy or sudden shock load. This type of pipe comes in various lengths, depending upon the trade but is essentially from ten feet to twenty feet long.
Rigid pipe sections are connected by screw couplings. Each end of a pipe section has tapered male threads formed thereon. These threads are V-shaped to standardized dimensions having a slightly flattened crest and a slightly flattened base. Each end of a screw coupling has tapered female pipe threads which mesh with the male pipe threads. The mesh is quite good and because the threads are tapered, when they are joined they make a joint that is essentially hermetic, that is, a joint that does not require sealing agents such as mastic or pipe compound, although sealing agents may be used.
The difficulties associated with the use of rigid pipe become apparent when such pipe is to act as a conduit for electric cables. Because the pipe is thick walled, each section is highly priced. In addition, each section is difficult to handle because of its weight. Because the pipe ends are threaded, each section must be rotated in the field when it is connected to another length of pipe. The weight of the pipe section makes the rotation difficult. Another drawback is that, if a section of pipe or a fitting is damaged, either the pipe or fitting has to be cut to eliminate the same and replaced, or the entire layout must be unscrewed piece by piece until the damaged section or fitting is removed.
Moreover, since the screw couplings must mesh with male threads on adjacent pipe sections, the sections must be in perfect axial alignment or the threads will not mesh properly. As is often the case in field installations, or where replacements must be made to existing pipe systems, the pipe sections are not in perfect alignment and problems result in assembling the sections. A further difficulty is the fabrication cost involved because of the wall thickness.
The other kind of conduit which has been developed for sheathing electrical cables is light walled conduit. Light walled conduit was designed solely as a conduit for encasing electric cables to protect them from damage. Because it does not have to take large stresses, it is thin walled. Obviously, an advantage of light walled conduit is its lighter weight and its lower cost.
A disadvantage is that it cannot be used in instances where it might be subjected to heavy loads. In addition, because of its thin wall, this type of conduit can not be threaded. Compression couplings are used most often to join sections of light walled conduit. The ensuing joint cannot withstand substantial internal pressure or tensile stress. The advantage of compression couplings over the screw couplings used in conjunction with the rigid pipe is that if any section of tubing or any fitting is damaged, the coupling can be opened and the damaged pipe section or fitting can be pulled out transversely without removing the fittings or sections that were not damaged.
Recently, due to the scarcity of and resulting higher prices for steel, a medium walled pipe known as utility pipe or intermediate metal conduit was developed having a wall thickness less than the wall thickness of the rigid pipe but greater than the wall thickness of the light walled conduit. This intermediate metal conduit is heavy enough to take intensive loads and can be hermetically sealed at the joints.
Intermediate metal conduit has used screw couplings for joints, causing the same problems to arise in connection with this conduit as with the rigid pipe namely, the necessity of rotating conduit sections to make up joints or the necessity of unscrewing the sections one after another from an end of a run of pipe when an intermediate section or fitting is to be removed. Further the axial alignment problem inherent in the rigid pipe system is also present. It is toward elimination of these difficulties with intermediate metal conduit that the present invention is directed.